The Doctrine of signatures states that while Nature (or the Creator) has taken care to provide us with everything we need to keep ourselves well, She (or He, or It) has designed many useful herbs with clues to their use by making them, or part of them, look like the parts of the body that they are most useful for. That is, marking them with the "signature" of their proper use.

The Doctrine of Signatures was used by ancient physicians, including Dioscorides (Greek CE 40-80) who was a medic in the Roman army and wrote De Materia Medica, the pharmacopeia of choice for more than a millenia and Galen (Greek 129CE - 200ish CE), the word on anatomy for well into the Renaissance and had a great deal of influence on many of today’s health-related scientific disciplines.

The term, the Doctrine of Signatures, seems to have first appeared during the Renaissance era, with the publication of the book The Signature of All Things (1621) by Jakob Boehme (German) Though it was the alchemist Paracelsus(1494-1542) who first taught Doctrine of Signatures as a formal study to his students. The book Adam in Eden or Nature's Paradise (1657) by William Coles stated The mercy of God… maketh… Herbes for the use of men, and hath… given them particular Signatures, whereby a man may read… the use of them. further solidified the notion. Coles taught that walnuts were good for the head, because they look like a head, and brains and that St John's Wort would be soothing to the skin because it had pores like skin. In both cases, he was correct.

Many common names of plants bear the memory of the doctrine of signatures. The flowers of eyebright were said to resemble eyes, and to be useful for ailments of the eyes. The leaves of lungwort, which bears the modern botanical name Pulmonaria (referencing the pulmonary system, i.e. lungs) resemble lungs and were believed to treat diseases of the lungs.

The doctrine of signatures was not limited to Western healing but also existed and continues to exist in Eastern tradition. In a somewhat more complicated tradition, Chinese medicine corresponds colors and flavors in plants to the organs they are used to treat.

While there is no scientific evidence to back up the doctrine of signatures, there have been many coincidences.